Birth defects of the CNS such as anencephaly and spina bifida are among the most severe and at the same time poorly understood conditions which may affect an infant. While there are considerable data on the morphological characteristics of these defects there is little information on the mechanisms which produce them. The intent of the current investigation is to determine the role of the neuroepithe- lial basal lamina (BL) in neurulation, were it may determine the shape, orientation and interactions between neuroepithelial cells. Studies of the time course, pattern of deposition and synthesis of type IV collagen and fibronectin will be carried out and quantified at the ultrastructural level using frozen thin sectioning-immunogold staining techniques combined with in situ hybridization. To determine if the presence of these compounds indicate a functional role for them in neurulation. Cell BL interactions will be perturbed by microinjecting nanoliter quantities of monovalent antibodies or synthetic peptides to the cell binding region of these compounds into the neuroepithelium, mesenchyme, or into the region of secondary neurulation, and embryos allowed to develop in whole embryo culture for an additional 4-12 hours. Possible alterations in the composition of BL in neurological mutants of the mouse with a high spontaneous incidence of cephalic neural tube defects: Splotch; or defects of the caudal neural tube: delayed Splotch, t-w18; will also be determined. These studies will be combined with in vitro studies of the response of tailbud mesenchymeto these substrates, as a model of the events involved in secondary neurulation. This combined approach will allow the precise correlation of morphology and composi- tional/synthetic alterations of the neuroepithelial basal lamina during normal development, as well as in spontaneous and induced neural tube defects. These results should elucidate not only important cell biological questions regarding the role of the extracellular milieu in determining cellular polarity, cell-cell recognition and interaction, but ac the same time will provide valuable new data on the pathogenesis of developmental defects of the nervous system.